The present invention relates to a derailleur gear for a bicycle including a device for transverse displacement of the chains with respect to the rear wheel of the bicycle. This device could, for example, be a jack perpendicular to or oblique with respect to the plane of the rear wheel or a parallelogram linkage located in a plane parallel to the axis of the rear wheel of the said bicycle. One of the constituent elements of the device is fixed to the frame of the bicycle and is connected to a "support" parallelogram linkage located in a plane perpendicular to the axis of the rear wheel. One or more parts of the support parallelogram linkage serve as a vertically movable support for a spindle carrying a chain guide pulley or sprocket and an articulated cap or arm which supports a chain tension pulley.
In a known form of construction of this type of derailleur gear, two parallelogram linkages are used, one of which serves as support for the chain guide sprocket and the other of which serves for the transverse displacement of the said sprocket along the conical circumference of all the change-speed gears of the rear wheel. Thus, it is possible to ensure, on the one hand, due to a first parallelogram linkage, the displacement of the chain guide sprocket of the derailleur gear parallel or obliquely to the axis of the rear wheel of the bicycle, which displacement causes the chain to pass from one gear to another, and, on the other hand, due to a second "support" parallelogram linkage, the displacement of the guide sprocket in a direction having a component perpendicular to the axis of the rear wheel, which displacement permits the distance separating the axis of this chain guide sprocket from the rear wheel axis to be varied as a function of the diameter of the gear used in such a way that the sprocket is always in the immediate vicinity of the said gear. Of course, instead of the first parallelogram linkage, a jack can be used as a transverse displacement device, this jack thus being disposed by the side of all of the change-speed gears in such a way that its axis is parallel to the axis of the gears or to a conical generatrix defined by the gears.
This type of derailleur gear for a bicycle has the disadvantage that the rigidity of the means proposed to ensure the connection between the transverse displacement device and the support parallelogram linkage is not ensured in a manner which is sufficiently accurate to permit a really satisfactory functioning of the whole gear. Certain proposed derailleur gears of this type only have a single connecting screw to connect together two sides belonging to the first and to the second parallelogram respectively and there then arises, at the position of this connection, flexures which are a function of the chain tension and reaction forces exerted by various springs in this region which are incompatible with a geometric, stable and accurate positioning of the chain guide pulley.